Again, depending on your setup, you may be able to use a device like Apple's Airport Express to extend your wireless network. Just plug it in somewhere where it gets a decent signal from your main router but closer to your PS3 and set it up as an extension to your existing network.
I have one and it works great, especially handy as it has a usb port and an ethernet port as well so I can connect devices that are not wireless to it.
http://www.apple.com/airportexpress/
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